Virtual Memory is a storage allocation scheme in which secondary memory can be addressed as though it were part of the main memory. The addresses a program may use to reference memory are distinguished from the addresses the memory system uses to identify physical storage sites and program-generated addresses are translated automatically to the corresponding machine addresses.

The size of virtual storage is limited by the addressing scheme of the computer system and the amount of secondary memory is available not by the actual number of the main storage locations.

It is a technique that is implemented using both hardware and software. It maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory.

1. All memory references within a process are logical addresses that are dynamically translated into physical addresses at run time. This means that a process can be swapped in and out of the main memory such that it occupies different places in the main memory at different times during execution.

2. A process may be broken into several pieces and these pieces need not be continuously located in the main memory during execution. The combination of dynamic run-time address translation and the use of a page or segment table permits this.

If these characteristics are present then, not all the pages or segments need to be present in the main memory during execution. This means that the required pages need to be loaded into memory whenever required. Virtual memory is implemented using Demand Paging or Demand Segmentation.

Demand Paging

The process of loading the page into memory on demand (whenever page fault occurs) is known as demand paging. The process includes the following steps are as follows:

1. If the CPU tries to refer to a page that is currently not available in the main memory, it generates an interrupt indicating a memory access fault.

2. The OS puts the interrupted process in a blocking state. For the execution to proceed the OS must bring the required page into the memory.

3. The OS will search for the required page in the logical address space.

4. The required page will be brought from logical address space to physical address space. The page replacement algorithms are used for the decision-making of replacing the page in physical address space.

5. The page table will be updated accordingly.

6. The signal will be sent to the CPU to continue the program execution and it will place the process back into the ready state.

Hence whenever a page fault occurs these steps are followed by the operating system and the required page is brought into memory.

Advantages of Demand Paging

• More processes may be maintained in the main memory: Because we are going to load only some of the pages of any particular process, there is room for more processes. This leads to more efficient utilization of the processor because it is more likely that at least one of the more numerous processes will be in the ready state at any particular time.

• A process may be larger than all of the main memory: One of the most fundamental restrictions in programming is lifted. A process larger than the main memory can be executed because of demand paging. The OS itself loads pages of a process in the main memory as required.

• It allows greater multiprogramming levels by using less of the available (primary) memory for each process.

Disadvantages of Demand Paging

• It can slow down the system performance, as data needs to be constantly transferred between the physical memory and the hard disk.

• It can increase the risk of data loss or corruption, as data can be lost if the hard disk fails or if there is a power outage while data is being transferred to or from the hard disk.

• It can increase the complexity of the memory management system, as the operating system needs to manage both physical and virtual memory.

Page Fault Service Time

The time taken to service the page fault is called page fault service time. The page fault service time includes the time taken to perform all the above six steps.

Let Main memory access time is: m
Page fault service time is: s
Page fault rate is : p
Then, Effective memory access time = (p*s) + (1-p)*m

Swapping

Swapping a process out means removing all of its pages from memory, or marking them so that they will be removed by the normal page replacement process. Suspending a process ensures that it is not runnable while it is swapped out. At some later time, the system swaps back the process from the secondary storage to the main memory. When a process is busy swapping pages in and out then this situation is called thrashing.

Thrashing

At any given time, only a few pages of any process are in the main memory and therefore more processes can be maintained in memory. Furthermore, time is saved because unused pages are not swapped in and out of memory. However, the OS must be clever about how it manages this scheme. In the steady state practically, all of the main memory will be occupied with process pages, so that the processor and OS have direct access to as many processes as possible. Thus when the OS brings one page in, it must throw another out. If it throws out a page just before it is used, then it will just have to get that page again almost immediately. Too much of this leads to a condition called Thrashing. The system spends most of its time swapping pages rather than executing instructions. So a good page replacement algorithm is required.

In the given diagram, the initial degree of multiprogramming up to some extent of point(lambda), the CPU utilization is very high and the system resources are utilized 100%. But if we further increase the degree of multiprogramming the CPU utilization will drastically fall and the system will spend more time only on the page replacement and the time taken to complete the execution of the process will increase. This situation in the system is called thrashing.

Causes of Thrashing

1. High degree of multiprogramming: If the number of processes keeps on increasing in the memory then the number of frames allocated to each process will be decreased. So, fewer frames will be available for each process. Due to this, a page fault will occur more frequently and more CPU time will be wasted in just swapping in and out of pages and the utilization will keep on decreasing.

For example:
Let free frames = 400
Case 1: Number of processes = 100
Then, each process will get 4 frames.

Case 2: Number of processes = 400
Each process will get 1 frame.
Case 2 is a condition of thrashing, as the number of processes is increased, frames per process are decreased. Hence CPU time will be consumed just by swapping pages.

2. Lacks of Frames: If a process has fewer frames then fewer pages of that process will be able to reside in memory and hence more frequent swapping in and out will be required. This may lead to thrashing. Hence a sufficient amount of frames must be allocated to each process to prevent thrashing.

Recovery of Thrashing

• Do not allow the system to go into thrashing by instructing the long-term scheduler not to bring the processes into memory after the threshold.

• If the system is already thrashing then instruct the mid-term scheduler to suspend some of the processes so that we can recover the system from thrashing.

Performance in Virtual Memory

• Let p be the page fault rate( 0 <= p <= 1).

• if p = 0 no page faults

• if p =1, every reference is a fault.

Effective access time (EAT) = (1 -p)* Memory Access Time + p * Page fault time.

Note: Page fault time = page fault overhead + swap out + swap in +restart overhead

The performance of a virtual memory management system depends on the total number of page faults, which depend on “paging policies” and “frame allocation”

Frame Allocation

Several frames are allocated to each process in either static or dynamic.

• Static Allocation: The number of frame allocations to a process is fixed.

• Dynamic Allocation: The number of frames allocated to a process changes.

Paging Policies:

• Fetch Policy: It decides when a page should be loaded into memory.

• Replacement Policy: It decides which page in memory should be replaced.

• Placement Policy: It decides where in memory should a page be loaded.

Advantages of Virtual Memory

• Increases the degree of multiprogramming: You can run many applications at once without buying more memory RAMs.

• Data Sharing: Data that is common can be shared between memory.

• Avoids Relocation: The code in the physical memory can be accessed whenever required without relocation of the code.

• Increases memory: Users can run larger applications in systems that cannot support further RAM. Thus, it can run applications that are larger than the physical memory.

• Increases effective use of CPU: Since more processes can be maintained in the memory, the CPU is more effectively used. Each page is stored on the disk while it's in use, and further removed.

Disadvantages of Virtual Memory

1. Makes the system slower: Swapping takes time affecting the system's speed and making it slower. This happens as it takes more time in switching between applications.

2. Lesser Hard Disk Space: Virtual Memory takes up storage space that could otherwise be used for long-term data storage.

3. Reduces System Stability: It negatively affects the overall performance of a system.